Our invention relates to lasers in general, and in particular to a metal ion laser utilizing negative glow discharge in an atmosphere of an inert gas such as helium (He) for the vaporization of a metal such as cadmium (Cd). The metal ion laser in accordance with our invention are particularly notable for the facilities for protecting the Brewster windows from contamination by the vaporized metal.
Metal ion lasers are capable of emitting multicolor beams by virtue of the intense excitations offered thereby. He-Cd ion lasers, for example, have so far been observed to emit 12 different color beams including those of the primary colors. The metal ion lasers are definitely suprior in this respect to liquid or solid lasers. There have, however, been some problems left unsolved with metal ion lasers.
The metal ion laser in general has a tubular, elongate housing with its opposite ends closed by Brewster windows. The midportion of this laser housing is equipped with heaters for heating the anodes and the metal chambers containing a source of metal ions. Therefore, during the operation of the laser, its midportion is heated to a significantly higher temperature than are its opposite end portions including the Brewster windows. Such temperature gradients in the longitudinal direction of the laser inevitably invite the flows of the vaporized metal toward its opposite ends and the consequent vapor deposition on the surface bounding the end portions. The metal vapor deposition on the Brewster windows is particularly objectionable because the Brewster windows when so contaminated causes a gradual decrease in the output radiation of the laser.
We are aware of a conventional solution to this problem. It suggests the provision of a pair of secondary anodes adjacent the ends of a hollow cathode nested in the midportion of the laser housing. The secondary anodes are intended to "blow" the metal vapor back into the hollow cathode with a view both to the prevention of Brewster window contamination and to a stronger laser oscillation. This known solution is per se well calculated to accomplish the purposes for which it is intended. We do, however, object to the undue deposition of the metal vapor, when it is blown back by the secondary anodes, on the end portions of the hollow cathode. The accumulation of the deposited metal gradually decreases the inside diameter of the cathode end portions, resulting in a correspond decrease in laser output.